Electron tube alarm device



June 28, 1955 V s E 2,712,130

ELECTRON TUBE ALARM DEVICE Filed July 11, 1949 J EMT #1 lnmm I IIIII I71 I 7, I 7t. Pea 20. 25/ i i J UPPI. y I I I l IN V EN TOR. 12/6 /4206'. J 1 44606 4 True may nited States Patent Ofiice 2,712,130 PatentedJune 28, 1955 ELECTRON TUBE ALARM DEVICE Richard E. Sprague, RedondoBeach, Calif, assignor to Northrop Aircraft, Inc., Hawthorne, Calif., acorporation of California Application July 11, 1949, Serial No. 103,985

Claims. c1. amp-24s This invention relates to electronic alarm devices,and more particularly, to such a device for detecting and indicating alow impedance between the heater and cathode of an electron tube or abank of electron tubes.

In electronic devices using a considerable number of electron tubes, thechances of a tube failure occuring are, of course, multiplied over usesinvolving only a few tubes. Miniature tubes are commonly used in suchdevices, to substantially reduce the overall size. It has been foundthat, in certain of these miniature tubes, the spacing between heaterand cathodeis so small that the shunt impedance between these twoelements is more liable to decrease or cause an actual short circuit,sometimes intermittently, and consequently, such failures are expectedto occur at intervals.

It is an object of the present invention to provide a means whereby aheater-to-cathode impedance of abnormally low value, of one tube in abank of electron tubes, can be detected and either a visual or auralsignal given.

Briefly, this invention comprises a voltage divider circuit connected toalter the grid voltage of an indicator tube so as to cause an alarmdevice to be actuated.

In the accompanying drawings, shown by way of illustration and notlimitation:

Figure l is a complete wiring diagram of the present invention, showingtwo banks of electron tubes each connected in parallel with a separateheater voltage source.

Figure 2 is a wiring diagram showing the invention as applied to a bankof tubes operating from a single heater voltage source.

Referring directly to Figure l for a detailed description of myinvention, two filament transformers 1 and 2 are arranged with the lowerend of one secondary winding 4 connected to the upper end of the othersecondary winding 5 at a tap 6. An A. C. supply voltage is impressedacross the respective primary windings, and a bank A of electron tubes 7is connected with heaters 9 in parallel with one secondary winding 4,while a second bank B of similar tubes '7 is similarly connected to theother secondary winding 5. The heater supply voltage might also beobtained from a single transformer having a secondary windingcenter-tapped to provide the proper voltage on each side of the center.The tubes 7 may be any indirectly heated cathode type tube, having acathode 10, and may include an anode 11 and one or more additionalelements 12.

A neutral wire 14 connected to the secondary tap 6 forms a common leadfor one side of each heater circuit. The cathodes of the tubes 7 areconnected directly, or through a relatively low impedance, to thenegative side of the anode circuit, shown in Figure l by a ground symbol15.

A negative D. C. voltage, E, from a battery, for example, is applied tothe neutral wire 14 through a relatively high value resistor 16. Alsoconnected to the neutral wire 14 is a grid 17 of a triode tube 19. Acathode 10a, or filament, as the case may be, in triode 19 is preferablyconnected to the negative side of the same anode circuit as mentionedbefore. A plate element 20 of triode 19 is connected to a parallelcombination of a condenser 21 and a relay 22 and then to a loadresistance 23 to which is connected the positive end of the anodecircuit voltage supply.

In normal operation, the resistor 16 will have no current flowing in it,since the heaters 9 are not connected to the cathodes it) of theelectron tubes 7, and the heater circuit is floating with respect toground potential and every part of the anode circuit. Hence, theconstant voltage E is impressed on the neutral wire 14 and also on thegrid 17 of the triode 19. The voltage E is sufiicient to cause triode 19to be in a non-conducting condition, but not so large as to exceed themaximum heaterto-cathode voltage rating of the electron tubes 7 in banksA and B. Since the neutral Wire 14 is connected to the secondary tap 6as well as to one end of each heater 9, substantially no A. C. voltageappears on the neutral wire or at the grid 17.

When the heater-to-cathode impedance of one or more of the tubes 7 inbank A or B becomes less than a predetermined amount, a current ofelectrons will pass from E, through resistor 16, through the heater tocathode, to ground, which is the opposite end of the voltage source E.This current flowing through resistor 16 now causes the voltage on theneutral wire 14 to be more positive than E and thereby allows triode 19to conduct when a certain value of grid voltage is reached. When triode19 passes a suflicient plate current, the relay 22 is actuated to causea warning light 24, for example,

to be illuminated, thus giving the alarm signal.

Condenser 21 is preferably provided for the purpose of stabilizing relayaction and passing any A. C. current which might be present.

By adjusting the voltage -E, the amount of heater-tocathode impedancedrop necessary to actuate the relay 22 can be varied considerably. Anaural indicator such as a buzzer, for example, or other warning devicemay be employed instead of the warning light 24, if desired.

Thus it is apparent that the actuation of the warning device indicates aheater-to-cathode leakage or short circuit in one of the banks of tubes7.

A slight modificationof the signal circuit, as shown in Figure 2,eliminates the requirement of two banks of r. tubes and a tapped ordouble-section heater voltage supply. Here, the tubes 7a are eachconnected across a single secondary Winding 4a by a power wire 25. Asimilar triode 19a and resistor 16a are used in conjunction with thevoltage E, and the resistor 16a is connected to one side of the powerwire 25. Therefore, an A. C. voltage is present at resistor 16a duringoperation, which would be undesirable to apply directly to the grid 17aof the triode 19a.

Also connected to power Wire 25 is a filter choke 26 in turn connectedto a filter capacitor 27, the junction between the choke and capacitorleading to the grid 17a, and the opposite end of the capacitor 27connected to the negative side of the anode circuit of the tubes.

This choke-capacitor combination filters out the A. C. voltage appearingat the upper side of resistor 16a and causes a practically pure D. C.voltage to be impressed on the grid 1%, this voltage amounting to thevalue of E, during normal operation. Condenser 2111 also assists in thefiltering operation, when the triode 19a starts conducting, to preventrelay chattering, or intermittent operation. Otherwise, the operation issimilar to that of the device shown in Figure 1.

From the above description it will be apparent that there is thusprovided a device of the character described possessing the particularfeatures of advantage before enumerated as desirable, but whichobviously is susceptible of modification in its form, proportions,detail con- 3 struction and arrangement of parts without departing fromthe principle involved or sacrificing any of its advantages.

While in order to comply with the statute, the invention has beendescribed in language more or less specific as to structural features,it is to be understood that the invention is not limited to the specificfeatures shown, but that the means and construction herein disclosedcomprise the preferred form of several modes of putting the inventioninto effect, and the invention is, therefore, claimed in any of itsforms or modifications within the legitimate and valid scope of theappended claims.

What is claimed is:

1. An alarm device for indicating a heaterto-cathode short circuit inany of a plurality of electron tubes having a substantially commoncathode circuit and a common heater circuit, comprising voltage supplymeans having its positive side connected to said cathode circuit, asupply resistor connected between the negative side of said voltagesupply means and said heater circuit, said heater circuit having noother electrical path to said cathode circuit, a triode warning tubehaving a control grid connected to said heater circuit, and. indicatingmeans connected in the anode-cathode circuit of said triode, the

value of said negative side being sufficient to cause blocking of saidtriode during normal operation of said electron tubes, but notsuflicient to exceed the maximum safe heater-to-cathode voltage ratingof said electron tubes, whereby an abnormally low impedance between theheater and cathode of any of said electron tubes will 1 cause current toflow through said supply resistor to unblock said triode to actuate saidindicating means.

2. Apparatus in accordance with claim 1 wherein said electron tubes areconnected in two banks, each bank having its heaters connected inparallel across one of two separate heater supply sources connected inseries, one common heater supply wire leading from a junction betweensaid heater supply sources, and wherein said supply resistor and saidcontrol grid are connected directly to said common wire, wherebysubstantially no A. C, voltage is impressed on said control grid.

3. Apparatus in accordance with claim 1 wherein said indicating meanscomprises an electromagnetic relay adapted to actuate a warning means.

4. Apparatus in accordance with claim 1 wherein said electron tubes areconnected with said heaters in parallel across single heater supplysource, a filter choke i connected between said control grid and saidheater circuit, and a filter capacitor is connected from said controlgrid to said cathode circuit, whereby substantially pure D. C., fromsaid voltage supply means, is impressed on said control grid.

5. Apparatus in accordance with claim 1 wherein said electron tubes areconnected with said heaters in parallel across a single heater supplysource, and wherein filter means is connected between said control gridand said heater circuit, whereby substantially pure D. C., from saidvoltage supply means, is impressed on said control grid.

References Cited in the file of this patent UNITED STATES PATENTS2,292,159 Richardson Aug. 4, 1942 2,329,518 Dimond Sept. 14, 19432,390,788 Harder Dec. 11, 1945 2,448,026 Grant, Jr. et al Aug. 31, 1948

